Cell Division Processes: Mitosis, Cytokinesis, and Meiosis Explained

1. 5.1 Cell Increase and Decrease • Cell division increase the number of somatic cells (body cells) • Mitosis: Division of the nucleus • Cytokinesis: Division of the cytoplasm • Apoptosis: Programmed cell death

2. 5.1 Cell Increase and Decrease • The Cell Cycle • Orderly set of stages that occur between the time a cell divides and the time the resulting daughter cells divide

3. 5.1 Cell Increase and Decrease • The Cell Cycle • Interphase • G1 • S • G2 • Mitotic Stage • Mitosis and Cytokinesis

4. The Cell Cycle

5. 5.1 Cell Increase and Decrease • Control of the Cell Cycle • Internal and External Signals • Signaling proteins called cyclins increase and decrease as the cell cycle continues • Three Checkpoints • G1 • G2 • M

6. The Cell Cycle

7. 5.1 Cell Increase and Decrease • Apoptosis • Cells undergo programmed cell death when they cannot complete mitosis or in response to external signals.

8. Apoptosis

9. 5.2 Maintaining the Chromosome Number

10. 5.2 Maintaining the Chromosome Number • Terms: • Chromatin: tangled mass of threadlike DNA in a non-dividing cell • Chromosomes: condensed DNA molecules observed in dividing cells • Diploid (2n): Cells have two (a pair) of each type of chromosome • Haploid (1n): Cells have half the diploid number of chromosomes

11. 5.2 Maintaining the Chromosome Number • Overview of Mitosis • Nuclear division in which chromosome number stays constant • DNA replication produces duplicated chromosomes • Each duplicated chromosome is composed of 2 sister chromatids held together by a centromere • Sister chromatids are genetically identical • During mitosis, the centromere divides and each chromatid becomes a daughter chromosome

12. Chromosomes and Chromatids

13. Mitosis Overview

14. 5.2 Maintaining the Chromosome Number • Mitosis in Detail - Animal Cells • Prophase-nuclear membrane disappears, centrosomes migrate, spindle fibers appear • Metaphase-chromosomes line up at metaphase plate, associated with spindle fibers • Anaphase-centromeres divide, sister chromatids migrate to opposite poles, cytokinesis begins • Telophase-nuclear membranes form, spindle disappears, cytokinesis occurs

15. Mitosis in Detail - Animal Cells

16. 5.2 Maintaining the Chromosome Number • How Plant Cells Divide • Occurs in meristematic tissues • Same phases as animal cells • Plant cells do not have centrioles or asters

17. Mitosis in Detail - Plant Cells

18. 5.2 Maintaining the Chromosome Number • Cytokinesis in Plant Cells • Flattened, small disk appears between daughter cells • Golgi apparatus produces vesicles which move to disk • Release molecules which build new cell walls • Vesicle membranes complete plasma membranes

19. Cytokinesis in Plant Cells

20. 5.2 Maintaining the Chromosome Number • Cytokinesis in Animal Cells • Cleavage furrow forms between daughter nuclei • Contractile ring contracts deepening the furrow • Continues until separation is complete

21. Cytokinesis in Animal Cells

22. 5.2 Maintaining the Chromosome Number • Cell Division in Prokaryotes: Binary Fission • Prokaryotes have a single chromosome • Chromosomal replication occurs before division • Cell begins to elongate to twice its length • Cell membrane grows inward until division is complete

23. Binary Fission

24. 5.3 Reducing the Chromosome Number • Meiosis • Occurs in the life cycle of sexually reproducing organisms • Reduces the chromosome number • Provides offspring with a different combination of traits from that of either parent

25. 5.3 Reducing the Chromosome Number • Overview of Meiosis • 2 divisions, 4 daughter cells • Cells are diploid at beginning of meiosis • Pairs of chromosomes are called homologues

26. 5.3 Reducing the Chromosome Number • Overview of Meiosis • Meiosis I • Homologues line up side by side at equator-synapsis • When pairs separate, each daughter cell receives one member of the pair • Cells are now haploid

27. Meiosis

28. 5.3 Reducing the Chromosome Number • Overview of Meiosis • Meiosis II • No replication of DNA occurs in this division • Centromeres divide and sister chromatids migrate to opposite poles to become individual chromosomes • Each of the four daughter cells produced has the haploid chromosome number and each chromosome is composed of one chromatid

29. Meiosis

30. 5.3 Reducing the Chromosome Number • Overview of Meiosis • Fertilization • Fertilization restores the diploid number of chromosomes in a cell that will develop into a new individual.

31. 5.3 Reducing the Chromosome Number • Meiosis in Detail • Genetic Recombination Occurs in Two Ways • Crossing Over • Independent Assortment

32. Crossing Over

33. Independent Assortment

34. 5.3 Reducing the Chromosome Number • Phases of Meiosis I • Prophase I • Synapsis occurs, nuclear membrane breaks down • Homologues line up side by side and crossing over occurs • Metaphase I • Homologous pairs line up at metaphase plate such that maternal or paternal member may be oriented toward either pole

35. 5.3 Reducing the Chromosome Number • Phases of Meiosis I • Anaphase I • Homologous chromosomes (each still consisting of 2 chromatids) undergo independent assortment into daughter cells • Telophase I • Cytokinesis produces 2 daughter cells which are haploid

36. Phases of Meiosis I • Fig 5.12

37. 5.3 Reducing the Chromosome Number • Interkinesis - period of time between Meiosis I and Meiosis II • Phases of Meiosis II • Prophase II- • Cells have one member of each homologous pair • Metaphase II • Chromosomes line up at the metaphase plate

38. 5.3 Reducing the Chromosome Number • Phases of Meiosis II • Anaphase II • Centromeres divide and daughter chromosomes migrate • Telophase II • Nuclei form, cytokinesis

39. Phases of Meiosis II • Fig 5.13

40. 5.3 Reducing the Chromosome Number • Nondisjunction • The failure of paired chromosomes or chromatids to separate during cell division • Results in cells with an abnormal number of chromosomes • Trisomy 21 (Down syndrome) is an example

41. 5.3 Reducing the Chromosome Number • Genetic Recombination • Promotes genetic variability • Independent assortment of paired chromosomes during metaphase I • Crossing over in prophase I • Both assure that gametes will contain different combinations of chromosomes • When fertilization occurs, the resulting offspring will be genetically unique

42. Comparison of Meiosis to Mitosis • DNA replication occurs only once prior to either meiosis and mitosis • Meiosis requires two divisions, mitosis only one • Meiosis produces four daughter cells, mitosis produces two • Daughter cells from meiosis are haploid, those from mitosis are diploid • Daughter cells from meiosis are genetically variable, while those from mitosis are genetically identical

43. Comparison of Meiosis to Mitosis

44. 5.5 The Human Life Cycle • Requires both mitosis and meiosis • Meiosis in the female is called oogenesis • Meiosis in the male is called spermatogenesis • At fertilization, the resulting zygote divides by mitosis for the processes of growth and development • Mitosis is used for repair throughout life

45. Life Cycle of Humans

46. 5.5 The Human Life Cycle • Spermatogenesis • Begins at puberty and continues throughout life • Primary spermatocytes (2n) divide in meiosis I to form two secondary spermatocytes (1n) • Secondary spermatocytes divide in meiosis II to produce four sperm

47. 5.5 The Human Life Cycle • Oogenesis • Begins in the fetus • Primary oocytes are arrested in prophase I • At puberty, one primary oocyte continues the process of meiosis during each menstrual cycle • Primary oocyte (2n) divides in meiosis I to produce one secondary oocyte (1n) and one polar body • Division is unequal as secondary oocyte receives most of the cell contents and half the chromosomes

48. 5.5 The Human Life Cycle • Oogenesis • If the secondary oocyte is fertilized, meiosis II will proceed. • Another unequal division will occur, the egg receiving most of the cytosplasm. A second polar body is also formed. • The unequal divisions allows the egg to have all the cellular “machinery” it needs for embryonic development

49. Spermatogenesis and Oogenesis in Mammals

50. 5.5 The Human Life Cycle • Summary • Spermatogenesis and oogenesis both utilize meiosis • Spermatogenesis begins at puberty and continues throughout life • Spermatogenesis produces one sperm per primary spermatocyte • Results in production of many sperm • Oogenesis results in one oocyte and up to three polar bodies per primary oocyte • Divisions are unequal, ovum receives most cell contents • Oogenesis begins prior to birth, stops until puberty, then resumes in a cyclic pattern • Cyclic release of oocytes continues until menopause when the process stops